The endogenous opioid system may play an important role in hedonic homeostasis. Not only are opioids rewarding, but also blockade of opioid receptors, in the absence of exogenous agonists, produces dysphoria and is aversive. This suggests the presence of an endogenous opioid tone responsible for maintaining a normal hedonic state. Surprisingly little is known about the nature of this system. We propose to use the place-conditioning paradigm to determine the anatomical, neurochemical and receptor components of the opioids-mediated hedonic tone in mice. Our preliminary data in rats point to the ventral pallidum and amygdala as important sites mediating the conditioned aversive effect of naloxone. We will begin by extending such microinjection studies to the mouse model. Subsequently, these will be refined using a lentiviral vector to express the mu receptor in selected regions of mu receptor knockout mice in an attempt to reinstate naloxone conditioned aversion. This will allow us to not only locate the brain regions involved, but also, ultimately, using appropriate promoters, to begin to identify the types of neurons mediating aversion. Using opioid receptor knockout mice, we have identified a primary role for the mu receptor in mediating naloxone aversion. With further use of such mice we plan to investigate the role of the delta receptor, and further, mu-delta receptor interactions, in opioids-mediated hedonic homeostasis. Similarly, the availability of opioid peptide precursor knockout mice will allow us to identify the endogenous ligand(s) responsible for the opioid hedonic tone. Our preliminary data suggest that beta-endorphin is not important in this regard. The possibility that constitutive activity of opioid receptors is involved, particularly in the opioid-dependent state, will also be investigated using inverse agonists of mu and delta receptors identified by Component by Kieffer, and ultimately, a constitutively active delta receptor mutant mouse generated by that Component. Information garnered from these studies will increase our understanding of the role of the endogenous opioid system in mechanisms underlying substance dependence and relapse.